1. Field of the Invention
The present invention relates to a solid-state imaging device, and particularly to a solid-state imaging device which includes a photoelectric converting portion and a charge converting portion for converting charges generated at the photoelectric converting portion into a pixel signal, for example, a CMOS image sensor and the like.
2. Description of the Related Art
A CMOS image sensor is a solid-state imaging device including a plurality of pixels arranged in a two-dimensional array each having a photoelectric converting portion and a plurality of MOS transistors, so that a charge generated at the photoelectric converting portion is converted into a pixel signal and read. In recent years, such CMOS image sensor has attracted attention as an image capture element used for cameras for mobile phones, digital still cameras, digital video cameras and the like.
In image sensors, charges are generated by photo-electrically converting light at a photoelectric converting portion, for example, photodiode. In addition, electrons/holes may be generated due to a temperature change. These electrons/holes are termed dark current. Dark current is a main source of noise in such image sensor and it may influence image quality. Also, dark current may occur not only in a photodiode but also in a floating diffusion portion. Dark current in typical image sensors changes depending on a temperature and an accumulation time. Accordingly, when the dark current occurs, a signal output value in an optical black state, that is, in a state without exposure changes depending on a temperature and an accumulation time, so that a reference optical black level changes to affect contrast of images. As described above, dark current may be a main source of noise in such image sensors.
For example, in the case where a certain fixed signal level is used for a standard of A/D (analog-to-digital) conversion, dark current may cause a digital value of an optical black level to change depending on a temperature. Hence, a black level of a portion of an image obtained after image processing may be deteriorated to become gray or entirely black, causing an image to have unstable contrast.
Accordingly, most of image sensors include an effective pixel (hereinafter referred to as an “aperture pixel”) in an effective pixel area and a black reference pixel (hereinafter referred to as an “OB (Optical Black) pixel”) that outputs an optical black level, formed on one device. The OB pixel has the same structure as that of the aperture pixel. However light is shielded in the OB pixel with a light-shielding film formed on a wiring, and therefore, an optical black level can be output for each frame by reading a signal of the OB pixel. Thus, even in the case where a temperature change causes a change in dark current, a reference contrast level can be estimated using the aperture pixel based on the black level of the OB pixel.
FIG. 1 shows a schematic diagram of a CMOS image sensor according to related art. As shown in FIG. 1, a solid-state imaging device 1 includes an imaging region 2, a vertical selection circuit portion 3 and a read circuit portion 4 disposed on the periphery of the imaging region 2. The imaging region 2 includes an effective pixel area 5 in which aperture pixels (effective pixels) 6 for capturing an object image are arranged and an optical black area 7 which is formed surrounding the effective pixel area 5 and in which light-shielded OB pixels 8 are arranged. Pixels including the aperture pixels 6 and the OB pixels 8 are regularly arranged in a two-dimensional array.
Each of the aperture pixels 6 and the OB pixels 8 includes, for example, a photodiode forming a photoelectric converting portion and a plurality of MOS transistors, for example, a transfer transistor, a reset transistor, an amplification transistor and a selection transistor.
The vertical selection circuit portion 3 includes a shift register, for example. The vertical selection circuit portion 3 selectively scans respective aperture pixels 6 and OB pixels 8 in the imaging region 2 sequentially one-line at a time in the vertical direction. Subsequently, the vertical selection circuit portion 3 supplies the read circuit portion 4 with signal charges generated at a photoelectric converting portion (photodiode) in each pixel in response to an amount of received light through a vertical signal line (not shown).
The read circuit portion 4 includes a horizontal signal line, a horizontal selection circuit, a column signal processing circuit, an output circuit and the like although not shown. Further, on the same chip, there is formed a control circuit. The control circuit generates a clock signal and a control signal to be referenced to operations of the vertical selection circuit portion 3, the read circuit portion 4 and the like based on a vertical synchronization signal, a horizontal synchronization signal and a master clock and inputs the clock signal and the control signal to the vertical selection circuit portion 3 and the read circuit portion 4.
FIG. 2 shows a cross-sectional structure on the line D-D′ that passes through the aperture pixel 6 and the OB pixel 8 in FIG. 1. As shown in FIG. 2, the aperture pixel 6 separated by an element isolating area 12 is formed on the effective pixel area 5 of a semiconductor substrate (for example, silicon substrate) 11 and the OB pixel 8 similarly separated by the element isolating area 12 is formed on the optical black area 7. The aperture pixel 6 includes a photodiode (PD) 13 as a photoelectric converting portion and a plurality of MOS transistors (a transfer transistor Tr1 alone is shown in FIG. 2). The transfer transistor Tr1 includes a semiconductor area as a floating diffusion (FD) portion 14, and a transfer gate electrode 15 formed through the photodiode 13 and a gate insulated film. The OB pixel 8 includes a photodiode (PD) 23 as a photoelectric converting portion and a plurality of MOS transistors (a transfer transistor Tr2 alone is shown in FIG. 2). The transfer transistor Tr2 includes a semiconductor area as a floating diffusion (FD) portion 24 and a transfer gate electrode 25 formed through the photodiode 23 and the gate insulated film.
A multilayer wiring 18 is formed above the semiconductor substrate 11 through an insulating interlayer 17. Further, in the optical black area 7, a light-shielding film 19 made of metal, for example, Al (aluminum) is formed above the multilayer wiring 18. It should be noted that a color filter, an on-chip lens and the like are formed on the light-shielding film 19 through a planarized film although not shown.
FIG. 3A shows a schematic planar structure of the photodiode (PD) 23 and the floating diffusion (FD) portion 24 of the OB pixel 8, and FIG. 3B shows a schematic planar structure of the photodiode (PD) 13 and the floating diffusion (FD) portion 14 of the aperture pixel 6, respectively. The size of the photodiode 23 in the OB pixel 8 is equal to that of the photodiode 13 in the aperture pixel 6, and the size of the floating diffusion portion 24 in the OB pixel 8 is equal to that of the floating diffusion portion 14 in the aperture pixel 6.
Using the above-mentioned arrangement, an image is read. Scanning operations to read image signals will be described below in detail. First, the OB pixels 8 and the aperture pixels 6 are sequentially selected and read in accordance with the horizontal synchronization signal through the vertical selection circuit 3. FIG. 4 shows an example of a signal output that is read. In this example, OB pixel lines on one side are made m1 including three lines (m1=3) and OB pixel lines on the other side are made m2 including two lines (m2=2) as shown in FIG. 1. Signal outputs of an OB pixel line and an aperture pixel line in one horizontal scanning period during a horizontal synchronization signal (XHS) are shown in an enlarged-scale.
With a vertical synchronization signal (XVS) triggered, an image of one frame is output during a period until the next vertical synchronization signal. A signal for one line is output with the horizontal synchronization signal (XHS) triggered in the period of the vertical synchronization signal. Regarding a signal output of one line, an OB pixel line outputs an optical black (OB) output level alone, but an aperture pixel line outputs optical black (OB) output levels a and b with a signal output c.
Typically, an output value of the OB pixel 8 read at the beginning of one frame is sampled and the output value of the OB pixel 8 is applied to the black level of the aperture pixel 6 within the frame based on the sampled result. Therefore, even in the case where a temperature changes suddenly, an amount of dark current due to the change of a temperature within one frame can be canceled.
According to the above-mentioned arrangement, even in the case where dark current that changes depending on a temperature and an accumulation time is generated in the aperture pixel 6, contrast of an image may be prevented from deteriorating with dark current. Specifically, since an amount of dark current is also measured in the OB pixel 8 under the same condition, contrast of an image on the aperture pixel 6 is determined by using the black level of the OB pixel 8 as a reference level.
However, in the case where the light-shielding film 19 is formed above the OB pixel 8, it may be difficult for an amount of dark current of the OB pixel 8 to be equal to that of the aperture pixel 6 due to the difference in a surface level between those pixels. The reason that the optical black level of the OB pixel 8 is used as a reference is based on an assumption that the amount of dark current of the OB pixel 8 and that of the aperture pixel 6 are equal. Therefore, in the case where the amount of dark current of the OB pixel 6 and that of the aperture pixel 8 are different from each other, the amount of dark current of the OB pixel 6 as the reference for the optical black level may not be applied to the aperture pixel 8. As a result, even in the case where a temperature may not change within one frame, image contrast on the aperture pixel 8 will be affected. A small difference between the amount of dark current of the OB pixel 8 and that of the aperture pixel 6 may greatly affect the image contrast in the case where a user intends to extend an accumulation time in order to capture an image of a dark object.
Japanese Unexamined Patent Application Publication No. H10-107245 (JP No. H10-107245 A) discloses a method of removing a photodiode from an OB pixel in order to control dark current in the OB pixel.